A turbocompressor's Surge Limit Line, displayed in coordinates of volumetric flow rate (Q.sub.s) and polytropic head (H.sub.p), can be difficult to characterize if the slope of the line is small; that is, nearly horizontal. And it can be especially difficult to characterize if the surge line exhibits a local maximum or minimum, or both. This is often the case with axial compressors having adjustable inlet guide vanes, and for centrifugal compressors with both variable inlet guide vanes and diffuser vanes.
The present-day method of transforming a Surge Limit Line to common invariant spaces, such as reduced flow rate and pressure ratio (q.sub.r.sup.2, R.sub.c) or reduced flow rate and reduced head (q.sub.r.sup.2,h.sub.r), does not diminish the characterizing problem.
Surge Limit Lines exhibiting local maxima and minima are not functions of pressure ratio or of reduced head since the relationship is not one-to-one for either surge line. For this reason, it is impossible to accurately describe them (even for constant equivalent speed) using the standard approach to construct an antisurge parameter, S.sub.s =f(R.sub.c)/q.sub.r.sup.2.